Metal structures are typically deposited on integrated circuits during their manufacture, by exposing the integrated circuit in a vacuum chamber to a source of the metal vapor. Vacuum evaporation of metals is achieved by placing the metal in a crucible within the vacuum chamber and heating the crucible until the metal reaches its boiling point. The liquid metal changes phase by absorbing the heat of vaporization from the heating source, and evaporates producing a cloud of the metal atoms in a vapor form which deposits on all exposed surfaces. Generally, refractory metal crucibles are employed for this purpose, which are composed of tantalum, platinum, or other suitable material. Crucibles made from such materials are relatively expensive and therefore it is important not to allow the crucible to be overheated after the contents of the crucible has evaporated. Not only does the excessive heating of the crucible produce a high temperature which physically destroys the crucible, but such inadvertently high temperatures will also cause the material of the crucible to itself evaporate, thereby contaminating the integrated circuit structures exposed.
One technique in the prior art to prevent the overheating of crucibles in a vacuum evaporation chamber, is to detect the end point of the evaporation of the contents of the crucible by visual inspection. Typically, a transparent celluloid film will be exposed to the evaporative metal cloud from the crucible on the inside of the vacuum chamber over a viewing window through the sidewall of the chamber. The operator of the chamber will observe the amount of deposition of metal vapor on the celluloid film at various times and when the operator judges that no more metal is being deposited onto the transparent film, the operator will turn off the heating source to the crucible. Not only is this form of inspection by the operator labor intensive, but it also produces an inaccurate and nonreproducible determination of the end point for the evaporation of the contents of the crucible. Since the quantity of material which is to be evaporated from the crucible is carefully calculated to enable the deposition of a desired thickness of resultant metal film on the integrated circuit, it is important to completely evaporate the contents of the crucible, and yet as was described above, it is also important not to extend the application of heat to the crucible, thereby destroying the crucible and possibly contaminating the resultant integrated circuit metal structures.